Ferrofluid-type seal apparatuses for use in sealing rotary shafts with single- or multiple-stage ferrofluid liquid O-ring seals about the shaft are well known. Ferrofluid seal apparatuses have been used as exclusion seals to protect one environment on one side of a shaft from contaminants in an environment on the other side of the shaft. Ferrofluid-type exclusion seals are particularly useful with computer-disc-drive spindles to prevent contamination in an environment from reaching a memory-disc area and where the exclusion seal does not require large pressure capacity. A standard ferrofluid exclusion seal presently employed comprises an annular, axially polarized ring-like permanent magnet adapted to surround a shaft and sandwiched between two identical pole-piece elements which are placed in a contacting magnetic-flux relationship with the one and the other polar ends of the permanent magnet. The pole-piece elements at the one end extend into a close, noncontacting relationship with the surface of the shaft to form a radial gaps therebetween. The ferrofluid is disposed and magnetically retained in the radial gaps on the insertion of a magnetically permeable shaft to form one or more liquid O-ring stages about the shaft which serve to form an exclusion seal.
Typically, for example, with a ferrofluid seal apparatus for use with a computer-drive shaft, the ferrofluid is shipped separately with the seal apparatus. The customer then installs the ferrofluid seal apparatus and inserts the shaft and then injects the ferrofluid inside the installed seal in order to activate the seal and to form a liquid O-ring seal about the shaft element inserted within the seal apparatus. This method of forming an exclusion seal is both time consuming and particularly subject to operator error in applying the proper quantity of the ferrofluid into the ferrofluid seal apparatus. The quantity of ferrofluid employed in the seal apparatus determines both the life and pressure capacity of the ferrofluid seal. It is therefore very desirable to supply a ferrofluid seal apparatus to a customer which already contains the proper quantity of ferrofluid, and which on the insertion of a magnetically permeable shaft into the seal apparatus, has the ferrofluid automatically filling the radial gap to provide the proper O-ring seal, thus in effect to be a self-activating seal.
The self-activating ferrofluid seal apparatuses have been disclosed in U.S. Pat. No. 4,252,328, and U.S. Pat. No. 4,252,353, both issued Feb. 24, 1981. The self-activating ferrofluid seal apparatuses described provide for self-activating, that is movement of the ferrofluid from a nonactivating to an activating sealing position within the radial gap upon the insertion of a magnetically permeable shaft. Such self-activating ferrofluid seal apparatuses are not wholly satisfactory, since the manufacturing cost of such self-activating seal apparatuses tend to be higher than for standard exclusion seals.
In addition, U.S. Pat. No. 4,357,024, issued Nov. 2, 1982, describes a ferrofluid seal apparatus designed for an extended ferrofluid seal life, and particularly for use as an exclusion seal with a computer-disc-drive shaft. The ferrofluid seal apparatus provides for a generally L-shaped pole pieces of the same or different width to form an interstage gap and a radial gap. In operation, the ferrofluid seal apparatus operates sequentially as a single-stage seal, and later, with the evaporation of the ferrofluid, a two-stage O-ring seal under each radial gap, thereafter a single-stage seal under one radial gap until seal failure, thereby providing a ferrofluid seal apparatus with an extended seal life.
It is therefore desired to provide for a new, low cost, self-activating seal of compact design which is easy to manufacture and assemble and to activate, and which has the characteristics extended seal life.